Recently considerable work has been directed to the operation of an induction machine as a self-excited generator. U.S. Pat. No. 3,829,758, entitled "AC-DC Generating System", which issued to the assignee of this application on August 13, 1974, teaches the use of an inverter-type circuit for exciting the generator. Since that time it has been found that the generator field can be modulated to provide a constant frequency a-c system. Such an arrangement is described and claimed in the application entitled "Modulated Induction Generator", filed Apr. 16, 1975, having Ser. No. 568,746, now U.S. Pat. No. 3,958,174 and is assigned to the assignee of this application. A subsequent step in the evolution of this technology was the provision of a controlled switching arrangement for both shorting the generator terminals to provide excitation for the induction generator field and periodically reversing the polarity of the voltage supplied to the load terminals, to provide a square wave alternating voltage from the same inverter-type circuit which excites the generator. This step is described and claimed in the application entitled "Variable Speed, Constant Frequency Induction Generator System", filed Nov. 28, 1975, Ser. No. 636,331, which issued as U.S. Pat. No. 3,982,170 on Sept. 21, 1976, and is assigned to the assignee of this application. This last-filed application pointed out that a generator having an unusual winding configuration, such as a plurality of closely-wound windings for each phase circuit, would be required to provide a constant-frequency, or controlled-frequency, quasi-square wave a-c voltage to energize a multi-phase load.
It is therefore a principal object of this invention to provide a switching system for both intermittently shorting the generator terminals to excite an induction generator and also to provide a constant (or controlled) frequency, quasi-square wave, three-phase a-c output voltage to a load, using only a conventional three-phase induction generator operating over a wide speed range.
A related and important object of this invention is to provide such a switching arrangement which is capable of full regeneration, and requires no large energy storage devices.
A corollary consideration of this invention is to provide such a power conversion system in which the frequency of the output voltage is independent of the generator frequency, and in fact may be either higher or lower than the frequency of the induction generator.
Another important object of the invention is to provide a nine-switch converter useful in direct ac-to-ac power conversion systems.
Still another important object of the invention is the provision of a power conversion system which receives a general quasi-square wave voltage at a given frequency, and converts it to another quasi-wave voltage at a different frequency.